The combination of Electrospray ionisation and mass spectrometry is a powerful technique for the analysis of organic compounds. Electrospray ionisation involves passing a solution of analyte in a volatile solvent through a capillary tube. The capillary tube is maintained at a relatively high potential with respect to a chamber surrounding the capillary tube and with respect to ground. A concentric flow of high velocity nitrogen is commonly provided at the tip of the capillary tube to aid the nebulisation process. The relatively high electric field which is generated penetrates into the liquid volume at the capillary tip and results in a partial separation of positive and negative electrolyte ions. When, for example, a positive potential is applied to the capillary tube then negative ions will be driven or attracted towards the inner capillary wall whilst positive ions will become enriched at the liquid-gas interface. Droplets with a net positive charge will then form at and be emitted from the capillary tip when the combined electrostatic and electrohydrodynamic forces exceed the liquid surface tension.
Heat may be applied to the charged droplets which will result in a further decrease in droplet radius at constant charge. A point is reached, known as the Rayleigh limit, wherein the coulombic repulsion of the charges exceeds the surface tension. The droplets then undergo fissions forming even smaller charged droplets or micro-droplets. The desolvation process continues until ions are liberated into the gas phase by the process of ion evaporation or charge residue. At least some of the resulting ions are then admitted into a mass spectrometer for subsequent mass analysis.
For liquid flow rates in the range 10-1000 nl/min Electrospray ionisation can usually proceed efficiently without the need to apply heat in the vicinity of the capillary tip. However, for mobile phase flow rates which are typically encountered in Liquid Chromatography Mass Spectrometry (“LC/MS”) which may be up to or in excess of 1 ml/min then it often becomes necessary to apply a significant amount of heat to the droplets emerging from the capillary tube in order to improve the ionisation efficiency and overall system sensitivity. In particular, it is known to surround the capillary tubes with a further (secondary) flow of nitrogen gas which has been heated. The amount of heat required to improve the ionisation efficiency increases with the flow rate and with the proportion of water in the liquid being ionised.
It is desired to provide an improved ion source.